1. Field of the Invention
The present invention relates generally to battery charging systems and more particularly to charging systems for preventing battery overcharge.
2. Background Art
Rechargeable batteries, for storing electrical energy, and battery chargers, for charging batteries and bringing the batteries back to a charged state, after the batteries have been depleted, have been known and are common. Typically, the batteries are charged after full or partial depletion by delivering energy to the batteries and reversing chemical processes within the batteries, by applying a voltage to the batteries, forcing current through the batteries, and, thus, restoring charge. A common charging technique is to apply a voltage source to the battery to be charged, which is greater than the battery voltage of the battery, and stop charging when the battery ceases to accept additional current. This almost always results in deleterious effects on the battery, reduces performance and battery life.
Batteries generally consist of two or more galvanic cells. Two electrodes of dissimilar materials are isolated one form the other electronically, but placed in a common ionically conductive electrolyte. Overcharge of the battery can lead to complicated and undesirable side reactions, in particular as they pertain to the decomposition of electrolyte. The latter can lead to gas production, which in turn leads to increased battery internal impedance. The battery with this increased battery internal impedance can quickly stray from optimum operating conditions. Additionally, overcharging promotes the growth of dendrites, which in turn leads to battery shorting. On other hand, present demands upon batteries call increasingly for greater power densities, so that undercharge is also to be avoided in any charging scheme.
Silver-based batteries typically have high energy densities, i.e., high energy to weight and volume ratios, an ability to deliver energy at relatively high current drains, and high reliability, making them excellent candidates for use in next generation technologies, as well as meeting current day energy storage and delivery demands. Thus, there is a need for a charging system that minimizes the deleterious effects of overcharging.
The charging of silver-based batteries is characterized by two plateaus, reflecting the two active oxidation states of silver. The first plateau occurs as silver is transformed to monovalent silver oxide (Ag2O) while the second plateau reflects the formation of divalent silver (AgO). Towards the end of charge, generally at approximately 90% of maximum capacity, the plateau transforms into a steeply rising curve and the battery begins to be overcharged. Consequently, a battery charging system that limits the maximum charging voltage and charging current is needed. The battery charging system should taper charge the battery, so as not to drive too much energy into the battery too fast, and, thus, prevent damage to the battery.
With the advent of more sophisticated and expensive battery systems, such as silver-based batteries and other high impedance batteries, the need arises for more advanced charging systems and methods, which prevent overcharging and damage to the batteries. This need becomes more important, especially for silver-based batteries and other high impedance batteries, which have high energy densities and require long term reliability. Such batteries may be used in spacecraft and in other applications, requiring no replacement or minimal replacement over extended periods of time. Thus, there is a need for devices and methods to facilitate charging such batteries to their maximum capabilities, with minimum or substantially no deleterious effects, and maximization of life of such batteries. The charging system should be inexpensive, easy to manufacture and use, small and light weight, durable, long lasting, reliable, and capable of being used in aerospace and defense applications.
Different battery charging systems have heretofore been known. However, none of these battery charging systems satisfies these aforementioned needs.
Different charging systems, using shunt regulators have been disclosed.                U.S. Pat. No. 5,821,733 (Turnbull) and U.S. Pat. No. 5,747,964 (Turnbull) disclose rechargeable batteries and battery charging systems for multiple series connected battery cells which include a plurality of shunt regulators, adapted to be connected in parallel with each of the cells. The voltage of each cell is monitored during charging. When a cell is fully charged, excess charging current is shunted around the fully charged cell to enable the remaining cells to continue to charge.        Turnbull shows different embodiments of his shunt regulators. In one of Tunrbull's embodiments, Turnbull simply shows shunt regulators, each in parallel with a battery cell. In another embodiment, Turnbull uses shunt regulators and field effect transistors, whose drain and source terminals are connected in parallel across each of the battery cells. Each shunt regulator is under the control of a voltage sensing circuit, which includes a differential amplifier which senses the actual cell voltage of the battery cell and compares it with a reference voltage, elsewhere in the charging circuit. In yet another embodiment, Turnbull uses a plurality of isolation switches to disconnect the battery cells from the charging circuit to prevent the battery circuit from discharging the cells when the battery charger is not being used.        U.S. Pat. No. 5,982,144 (Johnson et al) discloses a rechargeable power supply overcharge protection circuit with shunt circuits that shunt current about a battery or battery cell of a string of battery cells, when it is charged to a maximum charge limit. The shunt circuit includes shunt regulators connected across each battery cell.        U.S. Pat. No. 6,025,696 (Lenhart et al) discloses a battery cell bypass module having a sensor for detecting an operating condition of a battery cell, such as voltage or temperature, and a controller connected across the battery cell of a lithium ion battery, the controller then being operable to change to the conductive mode and thereby shunt current around the battery cell. The controller includes a voltage limiting operational amplifier operable for transmitting a voltage excessive output signal, when the input thereto exceeds a predetermined value, and a transistor having a predetermined gate voltage allowing bypass current flow, the transistor being responsive to the voltage excessive output signal from the voltage limiting operational amplifier to shunt current around the battery cell.        U.S. Pat. No. 4,719,401 (Altmejd) discloses zener diodes, each of which are shunted across each cell in a series connected string of battery cells.        
Different charging systems and methods, using plateaus and inflection points have been disclosed.                U.S. Pat. No. 5,642,031 (Brotto) discloses a battery recharging system with state of charge detection, that initially detects whether a battery to be charged is already at or near full charge to prevent overcharging. A state of charge test is first performed on the battery, by applying a current pulse and then observing the voltage decay characteristics which result, batteries which are initially nearly fully charged exhibiting a larger voltage decay than batteries which are not as fully charged. The result of this initial state of charge test is used to determine how to best terminate battery charging.        U.S. Pat. No. 4,392,101 (Saar et al) and U.S. Pat. No. 4,388,582 (Saar et al) disclose a method and apparatus of fast charging batteries by means of analysis of the profile of the variation with time of a characteristic of the battery, which is indicative of the variation in stored chemical energy as the battery is charged. The method comprises analyzing the profile for the occurrence of a particular series of events, preferably including one or more inflection points, which identify the point in time at which the application of a fast charge rate should be discontinued. Additional methods of analysis provide for termination or control of the charging current, upon the occurrence of other events such as limiting values on time, voltage or voltage slope, or a negative change in the level of stored energy. The variation of the characteristic with time is analyzed, preferably by measuring successive values of the characteristic, computing the slope and comparing successive slope values so as to identify inflection points and other significant events in the variation of the characteristic. Apparatus for performing these methods comprises a power supply and a microcomputer for analyzing the profile and controlling the power supply.        Saar and Brotto show a voltage-time curve, which can be separated into at least four distinct regions. Region I represents the beginning of the charging sequence just after the battery is initially attached to the charger and the charging begins. After the charging sequence passes through region I, the charging curve will enter a more stable region II. Region II is generally the longest region of the charging sequence, and is marked by most of the internal chemical conversion within the battery itself. Because of this, the voltage of the battery does not substantially increase over region II, and thus, this region represents a plateau region in the charging curve. At the end of region II is an inflection point in the curve, which represents a transition from region II to region III, and is noted by a point where the slope of the curve changes from a decreasing rate to an increasing rate. Region III is the region in which the battery voltage begins to increase rapidly with respect to time, thus, representing a region of rapid voltage rise. As the battery voltage increases through region III to its fully charged condition, the internal pressure and temperature of the battery also increases. When the effects of temperature and pressure within the battery begin to take over, the increase in battery voltage begins to taper off. This tapering off effect is noted as another inflection point and is also characterized by the sharp fall in the voltage derivative curve dV/dt. Region IV represents the fully charged region following the latter inflection point and including the charge termination target. The charging voltage only stabilizes at the charge termination target for a very short period of time. Consequently, if charging continues, the additional heating within the battery will cause the voltage of the battery to decrease and in addition may cause damage to the battery.        U.S. Pat. No. 6,215,312 (Hoenig et al) discloses a method and apparatus for analyzing an AgZn battery, which diagnoses the status of the battery having high and low voltage plateau states corresponding to its state of charge.        
Other fast charging devices and methods have been disclosed, some of which are complicated and involved.                U.S. Pat. No. 5,307,000 (Podrazhansky et al) discloses a method and apparatus, which uses a sequence of charge and discharge pulses. The discharging pulses preferably have a magnitude, which is approximately the same as the magnitude of the charging pulses, but which have a duration which is substantially smaller than the duration of the charging pulses. The discharging pulse causes a negative-going spike, which is measured and prompts the charging to stop.        U.S. Pat. No. 6,097,172 (Podrazhansky et al) discloses an apparatus and method for charging a battery in a technique wherein charge pulses are followed by discharge pulses and then first rest periods and other discharge pulses followed by second rest periods. Selected ones of the second rest periods are extended in time to enable a battery equilibrium to be established and the open circuit voltage of the battery to settle down and reflect an overcharging condition of the battery. By comparing the open circuit voltages measured during different extended second rest periods small voltage decreases are detected and used to determine an overcharging condition, such as when gases are generated and affect the open circuit voltage. Once overcharging is detected the battery charging is stopped. U.S. Pat. No. 6,232,750 (Podrazhansky et al) also discloses another battery charger, which rapidly charges a battery utilizing a bipolar waveform.        U.S. Pat. No. 5,204,611 (Nor et al) and U.S. Pat. No. 5,396,163 (Nor et al) disclose circuits in which rechargeable batteries and cells are fast charged by a controlled current, and substantially at a rate not exceeding the ability of the battery or cell to accept current. The resistance free terminal voltage of the battery or cell is detected during an interval when the charging current is interrupted, and compared against an independent reference voltage to control the charging current when a difference between the reference voltage and the sensed resistance free terminal voltage exists.        
Different charging systems and methods, using time as a factor in charging have been disclosed.                U.S. Pat. Nos. 6,137,268 (Mitchell et al) discloses a battery charging system in which current is averaged over a long time period (seconds) to determine the maximum average charging rate. When the integral of charging current over this long time period reaches the programmed maximum charge value for one period, current is simply cut off for the remainder of the fixed long period.        U.S. Pat. No. 6,215,291 (Mercer) discloses a control circuit, having a bandgap reference circuit, which minimizes the charging cycle time of a battery charging system, by maximizing the length of time that high constant charging current is applied to a discharged battery.        
Other charging devices, batteries, and methods have been disclosed, which still do not satisfy the aforementioned needs.                U.S. Pat. No. 5,166,596 (Goedken) discloses a battery charger having a variable-magnitude charging current source. U.S. Pat. No. 6,222,343 (Crisp et al) discloses a battery charger, which is capable of charging different types of batteries, a method for charging a battery, and a software program for operating the battery charger.        U.S. Pat. No. 5,387,857 (Honda et al); U.S. Pat. No. 5,438,250 (Retzlaff); U.S. Pat. No. 6,215,291 (Ostergaard et al); U.S. Pat. No. 6,037,751 (Klang); U.S. Pat. No. 5,089,765 (Yamaguchi); U.S. Pat. No. 4,113,921 (Goldstein et al); U.S. Pat. No. 5,049,803 (Palanisamy) U.S. Pat. Nos. 5,160,880 6,124,700 (Nagai et al); (Palanisamy) U.S. Pat. No. 4,745,349 (Palanisamy); U.S. Pat. No. 5,721,688, (Bramwell); U.S. Pat. No. 6,252,373 (Stefansson); U.S. Pat. No. 5,270,635 (Hoffman et al); U.S. Pat. No. 6,104,167 (Bertness et al); U.S. Pat. No. 3,708,738 (Crawford et al); British Patent Nos. GB2178608A (Yu Zhiwei) and 892,954 (Wolff); World Patent Nos. WO00/114848 (Simmonds) and WO01/47086 (Gabehart et al); French Patent No. FR2683093-A1 (Michelle et al); and        European Patent Application No. EP1076397A1 (Klang) each disclose other devices, batteries, and methods, which do not satisfy the aforementioned needs.        
For the foregoing reasons, there is a need for a charging system that minimizes the deleterious effects of overcharging. The charging system should limit the maximum charging voltage and charging current applied to the battery, and should taper charge the battery, so as not to drive too much energy into the battery too fast, and, thus, prevent damage to the battery. With the advent of more sophisticated and expensive battery systems, such as silver-based batteries and other high impedance batteries, the need arises for more advanced charging systems and methods, which prevent overcharging and damage to the batteries. This need becomes more important, especially for silver-based batteries and other high impedance batteries, which have high energy densities and require long term reliability. Such batteries may be used in spacecraft and in other applications, requiring no replacement or minimal replacement over extended periods of time. Thus, there is a need for devices and methods to facilitate charging such batteries to their maximum capabilities, with minimum or substantially no deleterious effects, and maximization of life of such batteries. The charging system should be inexpensive, easy to manufacture and use, small and light weight, durable, long lasting, reliable, and capable of being used in aerospace and defense applications.